Optical spectroscopy can provide quantitative information about the chemical composition and bonding configuration of molecular structures. Most molecules that comprise solid, liquid or gaseous samples have characteristic vibrational modes associated with their chemical bonds that can interact with photons. This results in optical absorption bands, mainly in the infrared spectral range, that are indicative not only of the chemical bond but also of the actual bonding configuration (i.e. X—Yn) and the local chemical environment. Non-contact infrared (IR) spectral reflectance or transmittance measurements probe these characteristic molecular vibrational modes of a sample to provide valuable information about the chemical bonding and composition in solids, liquids and gases.
Infrared spectroscopy, therefore, can yield more information than other methods which merely provide data about the relative content of X and Y in a sample. This methodology has been widely employed in laboratories using relatively large, bulk-optic dispersive NIR spectrometers and Fourier Transform Infrared (FT-IR) spectrometers as a diagnostic tool for the characterization of suitable samples and to assist process development for the synthesis and/or modification of materials. The table-top dispersive spectrometers generally consist of a free space arrangement of input optics, an input slit, mirrors to fold the optical path, a reflecting replicated grating, output optics, an output slit or detector array. For spectrographs using a single detector, additional scan motors are employed to vary the output wavelength impinging on the photo detector. FT-IR instruments employ an optical interferometer with a moving mirror in one arm of the interferometer to provide a spatial transform of the desired signal spectrum. This requires precise positioning of the movable mirror that entails an additional reference laser. Since the spectral information is not measured directly by the FT-IR instrument; substantial additional data processing is required to estimate the desired spectral information using mathematical transform techniques. The free space infrared spectrometers are sensitive to effects due to the ambient medium, such as optical absorption by water vapor. As a result, FT-IR and infrared dispersive spectrometers are generally purged using a supply of dry N2. The alignment of the free space optics is sensitive to temperature fluctuations and mechanical vibrations. This often requires additional costly environmental controls (i.e. temperature control, vibration damping) that add to the net system size and power consumption to assure the spectrometer performance.
There is a myriad of applications for IR spectroscopy in non-ideal conditions outside the laboratory that require performance characteristics comparable to a table-top FT-IR but in a more environmentally robust instrument with reduced size and mass for portability. Potential applications include, but are not limited to, the detection of biohazards in the working environment, blood analysis, analysis of soil samples and mineralogy by diffuse reflectance spectroscopy, monitoring of water and air quality, and the analysis of pharmaceutical powders and liquids. Many of these applications, including diffuse reflection measurements and biohazard detection, require a substantial system signal-to-noise ratio (SNR) to facilitate satisfactory resolution of the desired signal. For biohazard detection, detectivities in the ppm to ppb range are desirable, requiring very high SNR for a spectrograph system to be viable. For general diffuse reflection measurements, the return signal from an optically scattering target can be less than 5% of the incident illumination signal, yet a minimum SNR exceeding 1000 is desired for the spectral characterization of the diffusely reflected signal. Moreover, very intense illumination sources are undesirable as they can damage or alter the sample under investigation. Therefore, it is desirable to have a portable, lightweight infrared spectrometer capable of performance comparable to table-top dispersive or FT-IR instruments, but with improved tolerance to environmental perturbations.
There are several optical spectrometers in the prior art that attempt to remedy the problems of spectrometer size, weight, complexity and power consumption. Examples include Zeiss' Monolithic Miniature Spectrometer, or MMS 1 that is distributed by Hellma International Inc. The MMS 1 consists of a bulk silica cylinder with an integral reflecting imaging grating at the rear end of the cylinder. The input front face of the silica cylinder is coupled to a linear fiber array at the front input end and a silicon photodiode at the front output end. The use of silica limits the spectral range to the visible and NIR. Current spectral bandwidths in the NIR for the MMS spectrograph are limited to about 1 to 2.2 μm. The spectrometer design requires considerable bulk optical-quality glass that would be relatively expensive to duplicate in the infrared using typical IR transmissive materials such as ZnSe or Si. This would be further compounded if a broader spectral operating range in the infrared was required, since this would entail a much larger glass cylinder.
The LIGA-technique, involving X-ray lithography, electroforming and molding, has also been employed to fabricate miniature spectrometers for the VIS and NIR by Kernforschungszentrum Karslruhe GmbH, as represented by American Laubscher Corp. in the U.S. In the LIGA process, a X-ray sensitive polymer resist sheet, of the desired thickness of the waveguide core, is bonded onto glass or Si. A Ti mask is used to pattern the resist under exposure to X-ray synchrotron radiation. The exposed resist is removed by chemical stripping. This is employed to produce a molding insert. The mold can then be used to produce many copies by hot embossing. For the NIR, the spectrometer waveguide consists of an air gap between two plates containing vapor-deposited Au. This can provide attenuation below 0.3 dB/cm in waveguides exceeding 0.5 mm in thickness for an input NA=0.22. The loss is due to scattering and the finite reflectivity at the air/Au interface.
A resolution of about 10 nm was obtained over a 400 nm bandwidth in the LIGA NIR spectrometer near 1.6 μm. Extraneous stray light and noise is about 5% of the peak transmittance of the LIGA spectrometer, limiting its application to high signal applications. The typical waveguide core height is about 150 μm. Core heights up to 500 μm are possible, although there can be some degradation of the accuracy, verticality and smoothness of the grating elements. This limits the attainable optical luminosity of the LIGA spectrometer for low signal detectivity.
The theory and use of optical waveguides to provide optical confinement is well known and has been described in text books as early as the 1960's. In general, an optical slab waveguide consists of a sandwich structure of three basic layers; an upper cladding, an intermediate core layer, and a lower cladding. The main requirements for optical guiding are that the core layer be transmissive to the desired optical signal and that the refractive index of the core layer be larger than that of the upper and lower cladding layers. While the basic theory of optical waveguides is well documented, specific waveguide structures (core/cladding) with improved characteristics for a desired application could be the subject of new inventions. Various optically transmissive materials for the infrared spectral range are well known and documented, including Si, Ge, ZnSe, ZnS and sapphire.
Optical slab waveguides offer an important cost advantage over the use of bulk IR transmissive materials for the realization of integrated infrared spectrometers since the vertical optical confinement provided by the waveguide structure substantially reduces the amount of typically expensive IR material that is required to fabricate the spectrometer.
A miniature infrared spectrometer based on a slab waveguide structure employing a semi-insulating Si core and using the Rowland optical-layout geometry was reported in SPIE Proceedings of Infrared Technology and Applications XXII (vol. 2744, p. 684, 1996) in 1996 by the applicant. The main problem with this prior design was inefficient coupling of the optical output to linear detector arrays due to the curvature of the focal plane in the Rowland geometry.
The layout of most miniature spectrometers is based on a version of the Rowland geometry, as shown in FIG. 1. This concept employs a concave grating of radius R that minimizes the number of optical components required. In the classic Rowland geometry, the output focal plane lies along the Rowland circle of radius R/2. This is not suitable for coupling to a linear detector array.
More recently, James T. Daly, et. al (U.S. Pat. No. 6,303,934 B1) describe a miniature spectrometer based on a Si slab waveguide that consists of a flat front face containing an optical input portion, a diffraction portion with a machined chirped optical grating, and an exit portion with faces that are perpendicular to the plane of the slab waveguide. The rear portion of the slab waveguide contains a concave mirror. The spectrometer layout is based on the conventional and well known Ebert and Czerny-Turner, using an additional mirror to fold the optical path and reduce the length of the spectrometer at the expense of an increased width. This results in a relatively complex optical path that is sensitive to any non-idealities in the manufacturing process for a slab waveguide spectrometer.
The prior art slab waveguide spectrometer (U.S. Pat. No. 6,303,934 B1) employs a bare Si slab waveguide core with the surrounding medium or ambient acting as the upper and lower cladding. This is very disadvantageous for high-performance spectrometer operation for two main reasons. Firstly, stray ambient light can couple into the spectrometer through the air/core interface and contribute to the background signal. Secondly, without upper and lower cladding layers, the waveguide functions as a evanescent wave transmission cell. The optical signal within the Si slab penetrates slightly into the surrounding medium as it reflects from the air/Si interface. The optical signal thus interacts with the surrounding medium and can be absorbed by any condensates at the air/Si interface and infrared absorbing species within the surrounding medium. This is the basis of evanescent wave spectroscopy. Optical absorption by water vapor in the surrounding medium and condensation on the surface of the bare Si core can be particularly problematic due to the strong infrared absorption peaks. These factors can significantly obscure the spectral measurement of the desired optical signal.
The previous remedy (U.S. Pat. No. 6,303,934 B1) employs a special chirped grating to provide an approximately linear output focal plane at the output face of the slab waveguide. This is micro-machined onto the flat diffraction portion of the waveguide. Chirped gratings entail a grating period that varies along the width of the grating according to a prescribed formula. The concepts are well documented and well known to those skilled in the art. However, the variation in the grating period between adjacent gating elements can be quite small and comparable to the precision available to perform the micromachining. Therefore, the manufacturing of chirp gratings is complex and can be significantly more expensive than uniform gratings with a constant grating period.
Therefore, while some of the spectrometers in the prior art have attempted to remedy the problems of size, weight and complexity relative to typical laboratory grade infrared spectrographs and Fourier Transform Infrared (FT-IR) instruments, they do not provide a solution to the additional requirements of the spectrometer performance and signal detectivity relative to the laboratory grade instruments. This is especially critical for the miniature spectrometers to provide a viable solution in the infrared spectral range, spanning from 1.5 to beyond 12 μm, since the detectivity of typical uncooled infrared detectors such as PbSe or HgCdTe is several orders of magnitude less than that of Si visible detectors and InGaAs NIR detectors. In many of the applications where a compact, lightweight portable spectrometer system is highly desirable, such as biohazard detection, geological surveys of minerals in rock samples, analysis of contaminants in soil, water and air samples, and hyper-spectral planetary surveys for space exploration, the detectivity and SNR of the spectrometer system is of prime concern.
None of these prior art spectrometers permit fully monolithic operation with high SNR, low sensitivity to ambient conditions, and spectral sensitivity from 1 to beyond 12 μm using cooled detectors.